1. Field of the Invention
The present invention relates to gas turbine engines (turbo-prop or turbo-jet) used for aircraft propulsion and is particularly directed to an active control method and apparatus for the alleviation of the ground vortex ingested by the engine.
2. Description of the Related Art
Aircraft with engines mounted relatively close to the ground develop vortex activity during high-power, low speed and/or static ground operation. The ground vortex can generate high velocities near the ground, capable of dislodging foreign objects from the surface. This debris can become entrained in the airflow drawn into the engine inlet. The abrasion resulting from dust and dirt ingestion cause engine performance deterioration and reduced service life. Moreover, if the ingested debris includes large objects, engine failure with catastrophic consequences may result. The vortex can also create severe enough distortion to the core flow of the engine to result in engine surge.
The problems stemming from ground vortex ingestion are especially acute for large, heavy transport airplanes. Vortex ingestion greatly hinders the ability of these aircraft to land in austere fields and to perform essential ground maneuvers on unimproved terrain. Consequently, this can pose a severe limitation on their global reach.
In addition, some of the new commercial airplane concepts developed in recent years have a higher propensity for engine vortex formation due to unique propulsion-airframe configurations.
As will be disclosed below, the present invention provides the means to annihilate the ground vortex. Consequently, it prevents damage and/or premature engine removal caused by foreign object ingestion or engine surge.
The ground vortex phenomenon is schematically described in FIG. 1 (Prior Art). At low speeds the suction generated by the engine 1 results in the formation of a stagnation point 2 on the ground (like a vacuum cleaner effect). Usually, the ambient flow contains significant amounts of vorticity 3 (turbulence) due to gusts, ground turbulence, wake flow of neighboring aircraft components (i.e., wing, fuselage) and mixing of engine reverser plumes (when thrust-reversers are deployed). The mechanism of ground vortex formation is the amplification of the seed vorticity in the ambient flow due to the stretching of the contracting streamlines 4 approaching the engine inlet. This interaction results in a concentrated vortex 5 originating at the ground plane 6 and terminating inside the engine. Flow visualization indicates that the flowfield is unsteady, and the movement of the point of origination of the vortex on the ground is sporadic. Consequently, the vortex filament fluctuates over a significant portion of the lower inlet sector. The rotational flowfield induced by the ground vortex is the cause for kicked up dust and dirt, and it is a major source of engine compressor erosion. In addition, the tornado-like flow is capable of dislodging sizable foreign objects off the ground, causing Foreign Object Damage (FOD) to the engine. Another major concern is the inlet distortion created by the ground vortex, which may result in engine surge. As can be further seen in this figure, as the head wind velocity increases the streamlines become less stretched until there is little interaction between the engine inlet and the ground plane.
There have been several attempts to solve the engine problems stemming from ground vortex ingestion. In some of these approaches compressor bleed air is being used as a source for jets of air directed down and forward of the engine. The device described in U.S. Pat. No. 2,915,262, issued to H. J. Klein, uses a blow away jet that impinges on the ground, fans out forward of the engine and prevents the formation of the ground vortex. Other devices that utilize aft blowing jets underneath the engine cowl have been described in U.S. Pat. No. 3,599,429, issued to Bigelis et al. and U.S. Pat. No. 4,070,827, issued to Vanfleet et al. These jets produce entrainment underneath the engine, thus blocking the airflow from moving forward in the space between the bottom of the nacelle and the ground. These devices are used on commercially operated transports having wing mounted engines hung close to the ground which are generally operated over relatively clean surfaces.
A protective screen concept for alleviating engine FOD is described in U.S. Pat. No. 3,298,637, issued to Shao-Tang Lee. It requires the modification of the engine at the intake to include an encircling enlarged hollow conduit containing a series of holes around its circumference. Pressurized air is discharged through the holes to create an air screen to restrain or block dust that would tend to be thrown up by the engine at high power setting. This protective screen arrangement is a continuous flow system that creates a curtain of air to prevent the stagnation point from forming, requiring a large amount of bleed air and consequently draining the engine power. The creation of the air curtain will also cause a large amount of debris to become airborne, potentially causing more FOD problems. The configuration also requires a series of channels and doors to duct and direct the airflow, increasing the complexity and cost of the system.
A more recent attempt to solve the ground vortex is disclosed in U.S. Pat. No. 5,915,651, issued to Asaki et al. This device is operational in conjunction with the thrust reverser of a turbofan engine. During thrust reverser deployment the flow from a fan air bypass duct is being redirected downward and aft in order to create an air curtain for preventing inlet suction from underneath the engine. However, it has been demonstrated that in order for this device to obviate vortex activity, excessive amounts of airflow are needed, thus compromising the engine performance. Additionally, the high velocity air impinging on the ground blows dust and heavier objects up off the ground. This defeats the purpose of eliminating engine FOD, and also impairs crew""s visibility.
Another system to prevent ground inlet vortices is shown in U.S. Pat. No. 6,129,309, issued to Smith, et al. This system uses pulsating high pressure air to alternatively eject a control flow from two stationary nozzles mounted underneath the engine nacelle. The injection nozzles are directed towards the vortex stagnation point. The amount of air required to affect the inlet vortex is well within the bleed limits of the engine. However, tests have shown that steady blowing can have the same level of vortex alleviation as pulsing the jets. Additional testing would be required to determine if the pulsing jets would be more effective during crosswind conditions. This system also has the same potential problem as the previous mentioned systemxe2x80x94impingement on the ground may create FOD. However, the amount of flow is lower and relative distance from the ground is much higher which may limit the amount of FOD generated.
None of the prior techniques has proven to be entirely satisfactory in an operational sense. Many airplanes operate in a manner and environment where the engines are exposed to continuous operation at high power, at low speeds and statically, over ramp surfaces which are not clean. In particular, some airplanes use thrust reverse powerback for parking and ground maneuvering. Thrust reverser operation is a condition that is even more susceptible to ground vortex formation. One of the salient features of flows with increased propensity to ground vortex activity is the unsteady characteristic of the air motion. There are numerous factors contributing to the apparent randomness of the flow in a realistic and an uncontrolled environment. These include gusts with varying strengths and direction (time-variation of ambient flow), ensuing structural response of aircraft components (i.e., the flexing of the entire wing/engine assembly) and the unsteady turbulent mixing of the thrust reverser plumes.
The existing techniques target specific flow situations and they do not provide effective solutions for real life applications. Specifically, they constitute xe2x80x98point designxe2x80x99 solutions since they provide localized treatment of a ground vortex without consideration to the inherent problem of sporadic vortex motion. It is therefore highly desirable to develop a robust technique that addresses the underlying problem associated with vortex ingestion.
The present invention is an active system for the wide area suppression of a ground vortex generated by the engine of an aircraft. In a broad aspect it includes an actuator assembly in fluid communication with a fluid source; and, at least one nozzle assembly, including at least one movable nozzle. The movable nozzle is in fluid communication with the actuator assembly for receiving fluid from the actuator assembly. The actuator assembly controls the motion of the movable nozzle, wherein fluid is injected over a desired region relative to an inlet of the engine to disrupt the flow structure of a ground vortex, thus mitigating ground vortex ingestion. This ensures the operational health of the engine during airplane maneuvering on the ground. The fluid source is preferably the compressor of an engine of an aircraft.
Inlet vortices have plagued propeller and jet powered aircraft since their inception. Many solutions have been put forward with limited success in real life situations. Typically they have failed due to the complexity of the invention or by their limited capability to eliminate the inlet vortex. The present invention has addressed these shortfalls by: (a) considering the fundamental problem of unsteady characteristics of realistic inlet vortex flows, (b) keeping the invention simple by minimizing the number of moving parts, and (c) eliminating FOD due to the active system itself by preventing ground impingement of the control jet.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.